Conventionally, the structure of a linear motor used in a vacuum atmosphere is basically identical to that of a linear motor used in an atmospheric atmosphere.
The linear motor has a stator and movable element. The stator has a plurality of coils and a jacket which covers the coils and in which a refrigerant is supplied to cool the coils. When a current flows to the coils, the movable element moves relative to the stator. When the current flows to the coils, the coils generate heat. The heat is recovered by the temperature-controlled refrigerant flowing in the jacket.
In a conventional linear motor, the surface of the magnet of the movable element is coated with an epoxy resin for rust prevention. The jacket of the stator is made of a PEEK material or ceramic material to prevent an eddy current from being generated when the stator moves relative to the magnet of the movable element.
When the linear motor is used in a vacuum atmosphere as in a case wherein the linear motor is used by an electron beam exposure apparatus, the following technical problems arise.
(1) When heat enters a structure making up the linear motor or a structure around the linear motor, in the atmospheric pressure, the heat is released to the air, whereas in the vacuum atmosphere, the heat is released by only radiation. Accordingly, in the vacuum atmosphere, the temperature rise of the structure becomes larger than that in the atmospheric atmosphere. Consequently, the structure that receives heat tends to thermally deform. For example, when this linear motor is used by a precision positioning apparatus used in the vacuum atmosphere, the deformation of the structure caused by the temperature change causes deformation of a position measuring mirror or the like, leading to degradation in positioning precision.
(2) In the conventional linear motor, the jacket of the stator is made of a resin material or ceramic material. In particular, when the jacket is made of a ceramic material, it is difficult to degrease it. If fats and fatty oils attach to the jacket during machining or assembling the linear motor, the degreasing process is difficult. In the vacuum atmosphere, the water or oil content must be avoided from attaching to the structure in view of degassing. Therefore, in the linear motor used in the vacuum atmosphere, degassing of the fats and fatty oils attaching to it becomes an issue. Also, close attention must be paid so the fats and fatty oils or the like do not attach to the linear motor during machining or assembling.
(3) Furthermore, when the refrigerant for recovering the generated heat is supplied inside the jacket, for example, if a refrigerant such as a fluorine-based inert refrigerant with high insulating properties is used, static electricity is generated by friction of the refrigerant and jacket, and the jacket tends to be electrically charged easily. In an electron beam exposure apparatus that uses a linear motor in the vacuum atmosphere, when the structure of the jacket or the like is electrically charged, the charges influence exposure. For this reason, electric charges of the structure must be reduced.